Morphologic, genetic, and functional lines of evidence indicate that generation of Abeta peptides by proteolysis of amyloid precursor protein (APP) is a pathogenic process in Alzheimer's disease. Yet the responsible proteases remain undefined. Recent establishment of an in vivo co-transfection model to study APP processing by specific proteases has revealed that, among the known human lysosomal proteases tested, only cathepsin S functions to augment Abeta production by transfected cells. Moreover a form of APP with a transmembrane domain mutation, known to be linked to early-onset clinical disease, is more robustly processed to Abeta by cathepsin S than normal APP. Cathepsin S immunoreactivity is markedly upregulated in Alzheimer's disease brain and its distribution is consistent with the pattern of Abeta deposition. Thus the central hypothesis is advanced that cathepsin S, an inducible cysteine protease stimulated within human brain by injury and/or other as yet unknown factors, augments production of CNS Abeta and directly contributes to Alzheimer's disease pathology. This proposal is focused on elucidating the role of cathepsin S in Alzheimer's disease. Investigators of differing expertise have joined together for this purpose. Thus experiments are designed to test both the biochemical process whereby cathepsin S mediates Abeta production and the pathological and cellular mechanisms which underlie its expression in the brain. Forms of APP containing all the known mutations within the transmembrane domain of the protein will be expressed and analyzed in terms of processing to Abeta by cathepsin S and possible mechanisms which might account for altered processing by this enzyme explored. Experiments are detailed to define a cellular system which endogenously expresses APP and in which cathepsin S can be induced to determine if events in cells expressing less APP and cathepsin S, more like conditions in brain, recapitulate observations with transfected cells. In situ hybridization and immunocytochemical studies are planned on Alzheimer's disease. Down's syndrome, and primate brains of varying age to ascertain whether expression of cathepsin S is a relatively early or late event with respect to Abeta deposition and plaque formation. And mechanisms by which cytokine stimulation, cellular stress, or Abeta itself could regulate cellular APP processing by induction of cathepsin S will be defined. Results of experiments outlined above should clarify whether cathepsin S is an important enzyme in Alzheimer disease pathobiology and, if so, provide the molecular basis for drug design aimed at reducing Abeta production.